Solution mining of subterranean salt formations, such as carnallite beds or bischofite beds, has been performed in the past as a simple means of retrieving certain salts therein without having to face the problems associated with dry mining of such beds. Until now, aqueous solutions have been used for this purpose wherein water directed into a hole drilled into a bed forms a brine containing magnesium chloride, potassium chloride and sodium chloride and other soluble materials in solution. At saturation, the weight percentages of these three principal salts in the resultant aqueous brine are approximately 30% magnesium chloride, 3% potassium chloride and 10% sodium chloride.
When the above solution is lifted to the surface, it must be processed to separate the various salts from each other. This reqires considerable time and expense inasmuch as the brine must be processed by fractional crystallization techniques which are very costly to perform. Moreover, the latent heat of vaporization of water is approximately 1000 B.T.U. per pound so that considerable energy is expended in removing water from the salts and other solids in the brine in those climates where solar evaporation is impractical. Furthermore, because the saturation solubility of potassium chloride in the brine is relatively low (i.e., 3 wgt. %), aqueous solution mining is not the most practical way to obtain and utilize potassium chloride for potash.
In U.S. Pat. No. 3,833,709, a method is described for the recovery of magnesium chloride hexahydrate from carnallite ore subjecting the same to a solvent taken from the group comprised of the lower saturated monohydric aliphatic alcohols having 1 to 4 carbon atoms, such as methanol, ethanol, propanol, isopropyl alcohol, n-butyl alcohol, secondary-butyl alcohol, iso-butyl alcohol and tert-butyl alcohol. The solvent preferentially dissolves sustantially only the magnesium chloride hexahydrate content of carnallite; thus, substantially none of the other salts are dissolved by the solvent and the resulting solution is extremely rich in magnesium chloride hexahydrate.